The present invention generally relates to a mounting system for a turbine shroud and, more specifically, to a mounting system for a turbine shroud that provides radial compliance while minimizing looseness in the mounting system. The present invention also relates to methods for mounting a turbine shroud in a gas turbine engine.
Axial flow compressor or turbine rotor blade stages in gas turbine engines may be provided with shroud rings for the purpose of maintaining clearances between the tips of the rotor blades and the shrouds over as wide a range of rotor speeds and temperatures as possible. Blade tip clearances or clearance gaps that are too large reduce the efficiency of the compressor or turbine while clearances which are too small may cause damage under some conditions due to interference between the blade tips and the shroud ring.
The use of solid ring shrouds is common in gas turbines, but all of these applications must allow for thermal growth differences between the shroud and the engine case structure. In many applications this is accomplished by a rigid connection to the engine case with the flexibility of the shroud providing compliance. This generates stress and distortion in the shroud that is not desirable and may result in larger than desired tip gaps to prevent the blade tips from contacting the shroud. In other solid ring shroud applications thermal growth differences are accommodated by the use of a radially guided attachment. This method of attachment provides slots on the case and pins or tangs on the shroud arranged such that the shroud may grow relative to the case without building stresses. This type of arrangement must allow some clearance between the slots and pins or tangs to account for manufacturing tolerances and thermal growth of the slot and pin features. These clearances result in the shroud being loose in the case when assembled and reduces the ability to align the shroud to the center of blade tip rotation.
In gas turbine engines a tip clearance gap has to exist in order that the rotor blade tips keep clear of the shrouds under various operating conditions. It is usual to adopt a compromise whereby the tip clearance is large enough to avoid contact between the rotor blade tips and the shrouds but is made as small as possible for maximum efficiency. The positional accuracy of the inner surface of the shroud, relative to the blade tips is one of the variables that must be taken into account when making this compromise.
U.S. Patent Publication Number 2003-0202876 discloses a full ring low expansion ceramic to control the tip gap in a turbine shroud. As disclosed in the '876 publication, springs may be used to provide compliance for radial thermal growth and position control. By using a single spring of uniform stiffness, however, pins may be required to provide a positive stop, which, in many cases, may not provide the needed positioning control. The '876 publication uses three flats to prevent rotation in the event of a shroud rub. While these flats may impart local radial forces at three locations during a shroud rub, these forces may be insufficient to fully prevent rotation in the event of a shroud rub at higher shroud torque loads. Finally, the shroud of the '876 publication is axially positioned by two metallic radial plates with one edge exposed to the hot flow path. These plates may need to be slotted and cooled to prevent distortion and burning, resulting in additional machining time and expense.
As can be seen, there is a need for an improved mounting system for turbine shrouds and methods that provides radial compliance to limit the stresses experiences by the shroud due to thermal growth differences. Moreover, there is a need for an improved mounting system for turbine shrouds and methods that provide positional certainty during assembly, thereby avoiding the need for further tip clearances due to looseness during assembly.